1. Technical Field
The present disclosure relates to a control device for controlling a switching frequency of a quasi-resonant switching converter and a related control method.
2. Description of the Related Art
Switch-mode power supplies are affected by electromagnetic interference (EMI). EMI noise is generated when voltage and current are modulated by the switching converter comprised in the supply and this electrical noise can be transferred to the ac power line.
EMI noise affects the operation of some electronic systems by conduction; also, EMI induced noise on a power line may radiate or leak from the power line and affect other electronic equipment. Both conducted and radiated electrical noise may adversely affect or interfere with the operation of the electronic devices.
To address EMI related issues, regulations exist that define the maximum amount of EMI that can be produced by various classes of electronic devices and, in particular, by power supplies. Therefore, an important step in the design of a power supply is to keep EMI emissions within the limits specified by the applicable regulations.
EMI may be controlled in power supplies by adding input filters and snubbing the edges of the current and/or voltage switching waveforms. The extra components required to perform these tasks can undesirably increase the size and weight of the power supply. Further, they usually complicate the design process and increase the production cost: noise filtering components increase the cost and are often added on a trial-and-error basis during the final design process when EMI is found to exceed the compliance limits.
Frequency modulation (or jittering, or dithering as sometimes it is termed) is a technique that can facilitate the compliance of a switch-mode power supply with EMI emission regulations. In fact, on the one hand the emission of a switching converter is concentrated at the switching frequency and its higher-order harmonics. On the other hand, the EMI regulations envisage limits for the peak energy at any given harmonic, not for the total emitted energy. By modulating the switching frequency many side-bands are performed and the emission spectrum is scattered around these bands: this reduces the peak amplitude of the harmonics and makes it easier for them to stay below the EMI emission limits.
Normally, this technique is applied to power supplies in which the operating frequency of the switching converter is determined by an oscillator. Varying the oscillator frequency according to a given time profile will modulate the operating frequency of the switching converter and perform the above described spread-spectrum action on its EMI emission. This is extensively treated in the patent literature as well as the scientific literature.
Other types of switching converter exist where switching frequency is not determined by an oscillator. Examples of these converters are the old-fashioned ringing-choke converter (RCC) and the current transition-mode (TM) boost power factor corrector (PFC) pre-regulator and the quasi-resonant (QR) flyback converter. In these, the turn-on of the power switch is synchronized to the demagnetization of their magnetic storage device (inductor or transformer) and not by a clock signal provided by an oscillator. As a result, their switching frequency depends on the input voltage, the output load and the inductance associated with the magnetic device.
The dependence of the switching frequency on the input voltage provides these converters with a natural switching frequency modulation at twice the line frequency.
This is quite obvious in TM boost PFC pre-regulators, which operate directly from the rectified line voltage, so that their input voltage changes all the way from zero to the peak and then again to zero in a line half-cycle.
In QR flyback converters, as in most non-power-factor-corrected converters operated off the power line, the front-end stage is made up of a full wave rectifier bridge with a downstream capacitor filter, which provides an unregulated dc bus from the ac line. The filter capacitor is normally large enough to have a relatively low ripple at twice the line frequency, superimposed on the dc level. This ripple modulates the switching frequency at twice the line frequency with a depth depending on its amplitude.
This natural modulation at twice the line frequency provides an actual benefit in terms of EMI reduction, especially with the average (AV) detection method. Unfortunately, the effect is strongly dependent not only on the input voltage but also on the output power, which affect both the frequency deviation in a line half-cycle and the center frequency. As a general trend, the natural frequency modulation tends to reduce as the input voltage increases and/or the output load decreases. Additional, this low-frequency modulation is not very effective with the quasi-peak (QP) detection method.
In this class of converters, EMI reduction by frequency modulation can be improved by superimposing a higher-frequency forced modulation onto the low-frequency natural one; this provides a significant benefit also with QP detection.